Organization and Reactivity of Nanoparticles at Molecular Interfaces. Part I. Photoelectrochemical Responses Involving TiO2 Nanoparticles Assembled at Polarizable Water|1,2-Dichloroethane Junctions

The photoelectrochemical properties of TiO2 anatase nanoparticles were studied at the water|1,2-dichloroethane (DCE) interface under potentiostatic control. The interfacial concentration of the electrostatically stabilized particles can be effectively tuned by the Galvani potential difference and the pH of the aqueous phase. At pH values lower than that corresponding to the point of zero zeta potential, the particles assemble at the liquid|liquid boundary region upon positive polarization with respect to the Galvani potential in the organic phase. Upon band-gap illumination, photocurrent responses are observed in the presence of ferrocene in the organic phase. The potential and wavelength dependencies of the photocurrent unambiguously reveal that the photoresponses arises from the transfer of valence-band holes to the redox couple in DCE. The hole transfer is mediated via the generation of OHs• at the particle surface. On the other hand, photocurrents of the opposite sign were observed in the presence of TCNQ at pH's higher than 10. In this pH range, the particles are negatively charged, and the formation of the interfacial assembly takes place upon negative polarization with respect to the organic phase. The effect of the pH and the Galvani potential difference on the photocurrent suggests that heterogeneous charge transfer is in effective competition with recombination via OHs• as well as oxygen evolution at the particle surface. These studies open new possibilities for contactless photoelectrochemical and spectroscopic characterization of nanoparticles in solution in the presence of an electric field.